Experimental Determination of Dissolution Kinetics of Zr-Substituted Gd-Ti Pyrochlore Ceramics: Influence of Chemistry o
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Experimental Determination of Dissolution Kinetics of Zr-Substituted Gd-Ti Pyrochlore Ceramics: Influence of Chemistry on Corrosion Resistance Icenhower, J.P.1, Weber, W.J.1, Hess, N.J. 1, Thevuthasen, S.1, Begg, B.D.2, McGrail, B.P.1, Rodriguez, E.A.1, Steele, J.L.1, Geiszler, K.N.1 1 Pacific Northwest National Laboratory, PO Box 999, Richland, WA 99352, U.S.A. 2 ANSTO, PMB1, Menai, New South Wales 2234, Australia ABSTRACT The corrosion resistance of a series of zirconium-substituted gadolinium pyrochlore, Gd2(Ti1x Zrx)2O7, where x = 0.0, 0.25, 0.50, 0.75, and 1.00, were evaluated using single-pass flowthrough (SPFT) apparatus at 90ºC and pH = 2. The zirconate end-member, Gd2Zr2O7, has a defect fluorite structure, which distinguishes it from the face-centered cubic structure of the true pyrochlore specimens. In addition to the chemical variation, the samples include annealed, unannealed, and ion-bombarded monoliths. In the case of the titanate end-member, Gd2Ti2O7, the annealed specimen exhibited the least reactivity, followed by the un-annealed and ionbombarded samples (2.39x10-3, 1.57x10-2, and 1.12x10-1 g m-2 d-1, respectively). With increasing zirconium content, the samples displayed less sensitivity to processing or surface modification with the zirconate end-member exhibiting no difference in reactivity between annealed, un-annealed, and ion-bombarded specimens (rate = 4.0x10-3 g m-2 d-1). In all cases, the dissolution rate decreased with increasing zirconium content to the Gd2(Ti0.25Zr0.75)2O7 composition (1.33x10-4 g m-2 d-1), but the zirconate end-member yielded rates nearly equal to that of the titanate end-member. These results demonstrate that to achieve the greatest radiation and corrosion resistance in this series, the Gd2(Ti0.25Zr0.75)2O7 composition should be considered. INTRODUCTION Materials of the pyrochlore structural family exhibit a wide range of electronic, ferromagnetic, and ion conduction properties that makes them suitable for multifarious applications in the private and governmental arenas. Accordingly, these materials have enjoyed intense study and more than 450 varieties of the pyrochlore family have been synthesized [1]. Among their important properties is their resistance to chemical corrosion, which renders them useful for disposition of waste actinide materials. Of even greater importance, from a waste disposal stand point, is the discovery of radiation-resistant materials. Weber and Ewing [2] documented that substitution of Zr on the “B” site of pyrochlore (A2B2O7) results in a non-linear increase in radiation resistance. The ability of zirconium-rich compositions to withstand radiation damage stems from the configuration of oxygen atoms about the zirconium cation. Investigations probing the ordering of oxygen around Zr4+ have shown that considerable disorder, compared to oxygen surrounding Ti4+, is detectable over a range of compositions [3, 4]. The relative disorder appears to increase as the zirconate end-member composition is approached, suggesting that any atomic disp
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